(a) Field of the Invention
The present invention relates to an OFDMA (Orthogonal Frequency Division Multiple Access) FDD (Frequency Division Duplex) system. More specifically, the present invention relates to a method for configuring and allocating forward channels in an OFDMA FDD system.
(b) Description of the Related Art
In general, information transmitted forward by each base station provided in a cell in an OFDMA FDD system is classified as control data and traffic data. The information works as useful information for a base station, but it influences other neighboring base stations as interference.
For example, FIG. 1 shows an area influenced by the data transmitted by a base station BS1 in a first cell cn1 when first to third cells cn1, cn2, and cn3 are adjacent to each other. As to mobile stations MS1 and MS2 located in the first cell cn1, power intensity of the base station BS1 influencing the first mobile station MS1 is strong and surrounding interference influencing the same is weak since the first mobile station MS1 is located within the area of the first base station BS1, but power intensity of the base station BS1 influencing the second mobile station MS2 is relatively weak and surrounding interference influencing the same is relatively strong since the second mobile station MS2 is located on the border of the area of the first base station BS1, compared with those of the first mobile station case. In this state, when a mobile station located in the second cell cn2, i.e. a mobile station MS3, receives information from a base station BS2 by using very similar subcarriers in an area which is very close to the second mobile station MS2 located in the first cell cn1, information respectively provided to the second and third mobile stations MS2 and MS3 influences the other by interference which is generated among adjacent cells.
When respective users' priorities are defined by a scheduler for determining the priorities of user mobile stations, a channel allocation unit allocates subcarriers to the respective users following the defined priorities. In this instance, a channel allocation method for preventing interference between users uses a hopping pattern to regularly spread inter-cell interference or notify channel allocation information of a base station of other remaining base stations per period, and prevents other adjacent base stations from being allocated with repeated channels at the neighboring area.
A conventional paradigm will now be described as to an interference dispersion method using the hopping pattern and an interference avoiding method using a transmission time difference.
An exemplified interference-spreading method using the hopping pattern is disclosed by PCT (patent cooperation treaty) application number PCT/IL01/00681 entitled “Allocation of subcarriers to subscribers using Reed-Solomon codes”, to be described subsequently.
An R-S (Reed-Solomon) code based on the prime number 23 is used to generate a basic pattern, and respective cells use different codes which increase by one from the basic pattern to generate a hopping pattern, or all the cells respectively use different basic patterns to generate a hopping pattern.
When the R-S 23 basic pattern is {0, 5, 2, 10, 4, 20, 8, 17, 16, 11, 9, 22, 18, 21, 13, 19, 3, 15, 6, 7, 12, 14, 1}, respective numbers that configure the basic pattern set are numbers of subcarriers that configure respective groups when all the subcarriers are divided into 23 groups. When the first cell cn1 uses the basic pattern to allocate the subcarriers to the terminals for the 23 groups, the second cell cn2 increases the basic pattern of the first cell cn1 by 1, and uses the one-increased basic pattern. The third cell cn3 increases the pattern of the second cell cn2 by 1 and uses it as a hopping pattern. The respective hopping patterns used for the cells in this manner are as follows.
The set of the first cell pattern={0, 5, 2, 10, 4, 20, 8, 17, 16, 11, 9, 22, 18, 21, 13, 19, 3, 15, 6, 7, 12, 14, 1},
the set of the second cell pattern={1, 6, 3, 11, 5, 21, 9, 18, 17, 12, 10, 0, 19, 22, 14, 20, 4, 16, 7, 8, 13, 15, 2}, and
the set of the third cell pattern={2, 7, 4, 12, 6, 22, 10, 19, 18, 13, 11, 1, 20, 0, 15, 21, 5, 17, 8, 9, 14, 16, 3}.
The above-noted example is the method for using different hopping patterns to the cells to uniformly disperse inter-cell interference so that all the cells may use all the frequency ranges without superimposition at the transmission time. In the above, the increment of the pattern has been defined to be 1, and further, the set of patterns with the increment by n between the adjacent cells, that is, the set of patterns with great differences of subcarrier allocation patterns, can be selected.
The above-described method obtains a gain of dispersing the interference when load ratios of the surrounding cells are small, and cannot obtain the gain when the load ratios are large.
In a method for establishing transmission time differences for respective base stations and transmitting channel allocation information to other adjacent base stations, a base station that currently transmits channel allocation information notifies channel allocation information to be used at the area adjacent to other base stations, and other base stations allocate channels without using subcarriers repeated by neighboring base stations in consideration of interference used by the neighboring base stations. This TDMA-based information transmission and a channel-using method in the adjacent area are more effective since they essentially avoid interference compared to the interference dispersing method using the hopping pattern.
However, since the above-described control method and the method for transmitting data information with time differences previously considers the way other base stations have allocated the channels, and it is needed to re-allocate repeated channels, the repeated channels cannot be used, and hence the methods are ineffective in transmitting high rate packet information.